The present invention relates to a molten-metal treating vessel which comprises a vessel body having a drum directly connected to its bottom, the bottom of the drum adapted to be partially immersed in a bath of molten metal within a container to convey the melt out of the metal container into the treatment vessel and back again in a circulatory manner, so that during the course of refluxing the molten metal is treated in the manner desired, for example, for degassing or for refining the molten metal by the addition of one or more alloying elements or modifying additives such as desulfurizing and dephosphorizing agents, either added or blown into the bath, or by a blast of oxygen. In the following description and attached figures the conventional equipment is designated by a zero both in the discussion and in the drawings.
Conventional molten-metal treating vessels, for example, RH degassing vessels are, constructed as typically shown in FIG. 01(a)(b), so that the vessel body 01 is usually constituted by upper and lower sections 02, 03, which are separable for replacement because of different degrees of melting loss they suffer in operation, and two pipes 05, 06 for upward and downward flows are provided below a bottom wall 04 of the lower section 03 of the vessel, with immersion pipes 07, 08 detachably connected, respectively, to the lower ends of the upward and downward pipes 05, 06.
Another prior known vessel, for example, of the type described in Japanese Utility Model Publication No. 29526/71 and illustrated in FIG. 02(a)(b), has three molten-steel suction pipes 05 (or possibly more) and one discharge pipe 06 secured to the bottom wall of the lower section 03 of the vessel, with immersion or extension pipes 07, 08 removably connected to the respective pipes 05, 06.
Still another apparatus disclosed in Japanese Utility Model Publication No. 5205/66 is or a one-piece construction as shown in FIG. 03(a), with the bottom wall 04 of the lower vessel section 03 integrally secured to upward and downward pipes 05, 06 and immersion pipes 07, 08.
These treating vessels generally known in the art are designed to perform various treatments by immersing the lower portions of the immersion pipes into a bath of molten metal 010 in a ladle 09, conveying the molten metal upward with suction by vacuum into the lower vessel section 03, to a predetermined height from the bottom wall 04, blowing an inert gas such as argon into one of the intake immersion pipes 07, while maintaining the molten metal in the lower section at the predetermined level, and thereby producing an upward flow in the particular immersion pipe and a downward flow in the other immersion pipe, so that the molten metal in the closed circuit is continuously conveyed upward into the lower section, over the bottom wall, and downward out of the vessel during this procedure the molten metal may be degassed or subjected to one or more of the above-mentioned treatments simultaneously with the degasification. These treating vessel 01 are highly evacuated in operation, because it was previously believed effective for the purposes of degassing and other molten metal treatments.
However, conventional vessels such as 01 of the type described have presented the following problems:
(1) As a high degree of vacuum is created in the treating vessel 01, the molten metal rises into the lower section 03 of the vessel, eventually to a height h (see FIG. 01) of about 1.5 meters as measured from the normal free surface of the molten metal in the ladle 09. The height h.sub.1 of rise of the molten metal in the lower vessel section as measured from the bottom wall 04 depends both upon the lengths of the upward and downward pipes 05, 06 and the immersion pipes 07, 08 into the molten metal. On the other hand, it is believed desirable for the purposes of the molten metal treatments that the h.sub.1 usually be in the range of from 200 to 500 mm. Because of the lengths of the two pipes 05, 06 for the upward and downward flows and also of their associated immersion pipes 07, 08, the free surface or level of molten metal on the bottom wall 04 of the lower vessel section 03 is high above that of the bath in the container. This requires the use of a large-capacity evacuating means to draw the molten metal upward with suction using a high degree of vacuum. In addition, the high-vacuum treatment causes frequent splashes to considerable heights within the vessel 01, making it necessary to employ an otherwise unnecessary tall upper vessel section 02 for connection with the lower section 03. Moreover, the deposition of molten metal on the lining wall 011 on the inside of the vessel must be avoided by providing heater means adjacent those inner wall portions liable to be splashed. Consequently, the treating vessel body 01 that consists of the upper and lower vessel sections 02, 03 and the bottom wall 04 must be large in size and complex in construction.
(2) The cross sectional area of the bottom wall of the lower section 03 of the molten-metal treating vessel 01 is comparatively generally wider than the sum of the total sectional area of molten-metal flow passages inside the upward and downward pipes 05, 06 which are provided under the bottom wall. Because it has been conventionally believed that the wider the total liquid metal surface in the lower section 03 of the vessel 01 becomes, the more effective the molten-metal is degassed or subjected to other treatments.
However, in such a case, the exposed surface of the flow flux of molten metal formed between the immersion pipe 07 of the upward pipe 05 and the immersion pipe 08 of the downward pipe 06 over the surface of the bottom wall 04 accounts for a very limited percentage of the total liquid metal surface in the lower section 03 of the vessel. This means that the amount of molten metal retained in the lower section is so large that degasification or various other treatments that accompany the degasification cannot be efficiently carried out. Accordingly, it has been customary to increase the number of circuits or passes of the molten metal circulation or reflux, thus extending the treating time and lowering the metal temperature, with the result that the upward and downward pipes 05, 06, the immersion pipes 07, 08 connected thereto, and the bottom wall 04 and surrounding side wall 012 of the lower vessel section 03 have been seriously damaged by melting and consequently their operational life has been shortened. Accordingly it has been necessary to replace such parts at rather frequent intervals.
The apparatus according to Japanese Utility Model Publication No. 29526/71, as illustrated in FIG. 02(a), includes three or more suction or intake pipes. Since the required refractory lining adds greatly to the wall thickness of each such pipe, an increased number of pipes necessarily means an increased overall wall thickness. As a consequence, the sum of cross sectional areas of the flow passages for the molten metal is limited, and an adequate upward flow by suction of molten steel cannot be accomplished.
(3) The construction of the lower vessel section 03 is further complicated, as shown in FIGS. 01(a) and 02(a), with the upward pipe 05, downward pipe 06, and immersion pipes 07, 08, individually secured or connected to the bottom wall 04 of the treating vessel body 01, or is complex and difficult to construct, as apparent in FIG. 03(a) (b), as the upward and downward pipes 05, 06 and their immersion pipes 07, 08 are integrally formed and connected to the bottom wall 04 of the lower section 03. To extend their life, very frequent repairs are necessary with attendand labor costs and down time. Particularly, in the arrangements described in Japanese Utility Model Publication No. 5205/66 and illustrated in FIG. 03(a), the upper openings of the upward and downward pipes 05, 06 are directed away from each other, and therefore the side wall portions of the bottom section of the vessel 03 upon which the stream of molten metal impinge tend to become partly melted and damaged. To avoid this, the upper openings and the surrounding wall portions of the bottom must be kept a substantial distance apart. In addition, a refractory molding 041, for example, must be employed as the bottom wall 04 for directing the flow of liquid metal in opposite directions. For these reasons, the volume of the treatment vessel must inevitably be large in size.
(4) Further, as indicated above, such vessel bodies 01 for treating molten metal have limitations as to their construction and fabrication. For example, with the treating vessel of FIG. 02(a), which includes the downward pipe connected to the center of the bottom wall and surrounded by the plurality of upward pipes, it is necessary, as already pointed out, to use refractory linings of sufficient thickness that line the upward and downward pipes. This renders it impossible to attain the desired reflux velocity by maintaining the ratio of the total cross sectional area of the upward and downward flow passages 05', 06' to the area of the bottom wall 04 over the value discovered by applicants as discussed below. Furthermore, the undue increase in the number of molten metal circuits or passes prolongs the treatment time, resulting in low productivity, substantial temperature drop of liquid metal, increased damage caused by melting, and other problems of the type mentioned already.
A simplified molten-metal treating vessel known, which is contacted with exposed parts of molten metal surface in a container and to introduce addition agents into the metal, while covering the exposed part, at an ordinary pressure and in a non-oxidizing atmosphere. It is merely a cylindrical structure open at the lower end. Because it lacks the function of stirring the molten metal in the container, a gas nozzle or nozzles must be provided at the bottom of the container so that an inert gas may be blown into the bath for the agitation purpose. In addition, the cylindrical structure must be set in an immersion position such that the inert gas blown into the bath will force the slug sideways and the cylinder covers just the exposed and raised part of molten metal. These operational requirements extremely complicates the treating operation. Moreover, because the addition agents have to be introduced into the very narrow raised part of molten metal, it is important to blow a large volume of the inert gas into the bath for agitation over a long period of time in order that the additives be thoroughly added to and melted in the bath. The molten metal temperature naturally drops sharply during this procedure, which is a distinct disadvantage.
An apparatus of this type of construction is also used in a process for degassing molten metal in a low vacuum, for example, within a range from 10 to 300 mmHg as proposed by U.S. Pat. No. 4,152,140.
With such an apparatus one encounters the same problems as are involved in the degasification in a high vacuum, as already described.